Rel-10 of Long Term Evolution (LTE) supports aggregation of multiple carriers in a communication system. With carrier aggregation, a wireless device may receive information on multiple contiguous or non-contiguous carriers on a downlink and similarly send information on multiple carriers on an uplink. In Rel-11, additional carrier types, possibly having features that are only accessible by wireless devices compliant with Rel-11 and later releases, are under study. Such carriers will be aggregated with legacy Rel-10 carriers. In Rel-10, a new carrier type has to be aggregated with a legacy carrier type.
As illustrated in FIG. 1, it is possible to aggregate the new carrier, also denoted non-legacy carrier NLc, with a legacy carrier. With such non-contiguous carrier aggregation inter component-carrier interference is reduced by virtue of the frequency separation between the two carriers. The frequency separation also enables the use of respective filters in the receiving part to further suppress interference from one component carrier onto the next.
However, one difficulty arises in that different frequency bands are typically allocated or sold to different operators. The operator may then own certain frequency bands, but may be regulated to use a certain technology, or have to fulfill certain criteria set up by e.g. International Telecommunications Union (ITU) and/or governments. For e.g. such reasons many operators do not have the proper type of spectrum for deploying such non-contiguous carrier aggregation, with e.g. a narrow band legacy carrier and a wide band non-legacy extension carrier.
Further difficulties arise when trying to meet the desire of providing backwards compatibility, e.g. when aggregating a new carrier type with a legacy carrier type. When using contiguous carrier aggregation it is important that the signals from the two component carriers reach a receiving part with approximately the same received power, otherwise the receiver cannot properly tune automatic gain control settings. Further, for e.g. LTE, wherein cyclic prefixes are used, the two component carriers must also use the same cyclic prefix length if the component carriers are to remain orthogonal, and the signals must reach the receiver with approximately the same timing. Thus, if the carriers have different cyclic prefixes, they will not be orthogonal and will thus interfere with each other.
Carrier aggregation thus entails several challenges to be overcome.